1. Field of the Invention
The subject of the present invention is an exhaust device for a turbomachine such as an auxiliary power unit, a turbogenerator, a fan, a boost compressor, a turboprop or alternatively a turbomotor capable of driving, for example, a helicopter rotor shaft, the exhaust device being treated in such a way that its acoustic signature is reduced.
2. Discussion of the Background
It is known that turbomachines are equipped with an exhaust device which acts as a diffuser capable of slowing down the exhaust stream. An exhaust device of this kind generally comprises an outer casing, the shape of which is symmetric with respect to an axis. The precise shape of the exhaust device and its length along the axis of the exhaust stream may be determined so as to yield given performance bearing in mind, in particular, the static pressure at the inlet and at the outlet of the exhaust device.
One or more internal partitions defining several roughly concentric ducts for the exhaust stream to flow through have already been provided in exhaust devices for turbomachines, inside the casing. The multi-duct exhaust device thus produced has a smaller axial length for a given level of performance.
It is also known that in many applications it is desirable to reduce the noise generated by the turbomachines as far as possible. Acoustic treatment of the exhaust device has been envisaged for this. However, hitherto, the reduction in acoustic signature is considered as being insufficient. Document U.S. Pat. No. 4,109,750 describes a device for attenuating the sound of a turbomachine by means of the use of an acoustic material which can be applied to the walls of an internal partition. However, this application is to the detriment of the aerodynamic performance.
The object of the present invention is to improve the reduction in the acoustic signature of a turbomachine while at the same time making it possible to obtain an exhaust device with given aerodynamic performance. More specifically, the present invention makes it possible to improve both the aerodynamic performance and the acoustic performance of an exhaust device for a turbomachine, particularly a turbomotor for driving a helicopter rotor.
To this end, the turbomachine diffusion exhaust device according to the invention, which is of the multi-duct type with an annular inlet, comprises an outer casing shaped symmetrically overall with respect to an axis, and one or more internal partitions defining several roughly concentric ducts for the exhaust stream to flow through. In accordance with the present invention, an absorbent coating capable of absorbing some of the acoustic energy generated by the flow is provided on at least one surface of one internal partition and/or on the interior surface of the outer casing. The thickness of the absorbent coating is tailored along the length of the exhaust device to optimize the flow in the various ducts, while maintaining progressive diffusion and avoiding detachment near the stationary surfaces.
By virtue of this arrangement, the acoustic signature of the turbomachine is greatly reduced because it becomes possible to benefit from the increase in acoustically-treated surface area due to the presence of the internal partitions.
The absorbent coating is chosen not only according to its inherent absorption capability, characterized by its coefficient of absorption as a function of frequency, but also according to the characteristics of the aerodynamic flow (temperature of the gases in the exhaust and flow speed).
To optimize the aerodynamic performance, the area of the various ducts may advantageously vary along the length of the axis so as simultaneously to optimize the flow speed and the thickness of the absorbent coating, that is to say the acoustic performance.
The absorbent coating is arranged on the surface of the internal partition and/or the outer casing facing the flow.
The absorbent coating comprises an acoustic material which may be of the porous type or of the resonator type in a single layer or as multiple layers.
In the case of a porous material, this material may consist of agglomerated fibres (felt, glasswool or rockwool) or of an interstitial array produced by a collection of hollow microspheres. In the case of such a porous material, the absorbent coating further comprises an acoustically transparent wall which has the purpose of mechanically retaining the porous material. This wall may or may not be secured to the porous material.
When there is a central body along the axis of the device over most of its length, the said central body may also on its surface have an absorbent coating as defined hereinabove, and this further increases the acoustically treated surface area.
Likewise, if the exhaust stream is let out via a part which deflects the stream with respect to the inlet axis, the said deflecting part, which may be of any shape, may have an absorbent coating on its interior surface.